Germanium (Ge) Windows
Introduction:
Our topic is Germanium Windows, but first, let’s have a general view of the material Germanium. Single Crystal Germanium, referring to the germanium crystal without large angle grain boundaries and twinning, which contains a diamond structure, is a versatile material showing great capabilities and growing potential in various fields including semiconductor, fiber communication, aviation, solar cells, infrared optics, etc. The recent data shows that Infrared optics will take the largest portion of the growth of the demand for germanium in the future, attributing to the exponentially emerging demand both in the defense sector and the non-government sector.
The Material Properties of Single Crystal Germanium:
Transmission Range : 1.8 to 23 μm (1)
Refractive Index : 4.0026 at 11 μm (1)(2)
Reflection Loss : 53% at 11 μm (Two surfaces)
Absorption Coefficient : <0.027 cm-1 @ 10.6 μm (typical)
0.006 cm-1 at 5.6 μm (typical)
<0.005 cm-1 at 2.7 μm
Reststrahlen Peak : n/a
dn/dT : 396 x 10-6 /°C (2)(6)
dn/dμ = 0 : Almost constant
Density : 5.33 g/cc
Melting Point : 936 °C (3)
Thermal Conductivity : 58.61 W m-1 K-1 at 293K (6)
Thermal Expansion : 6.1 x 10-6/°C at 298K (3)(4)(6)
Hardness : Knoop 780
Specific Heat Capacity : 310 J Kg-1 K-1 (3)
Dielectric Constant : 16.6 at 9.37 GHz at 300K
Youngs Modulus (E) : 102.7 GPa (4) (5)
Shear Modulus (G) : 67 GPa (4) (5)
Bulk Modulus (K) : 77.2 GPa (4)
Elastic Coefficients : C11=129; C12=48.3; C44=67.1 (5)
Apparent Elastic Limit : 89.6 MPa (13000 psi)
Poisson Ratio : 0.28 (4) (5)
Solubility : Insoluble in water
Molecular Weight : 72.59
Class/Structure : Cubic Diamond, Fd3m
The Application of Germanium In the Field of Infrared Optics:
As an infrared optical material, germanium has the advantages of a high refractive index, wide infrared transmission range (Germanium is opaque to the visible portion of the spectrum, which implies it could block off the distraction of infrared devices), low absorption coefficient and dispersion rate, high chemical inertia(it is not susceptible to most of the acids and alkalis), etc. It has a Mohs hardness of 6.0-6.5, which implies it is not too hard and easy to process. These virtues make germanium an excellent material for manufacturing the windows, lenses, prisms, and filters of infrared thermal cameras, infrared radars, and other IR devices; germanium of high purity or germanium-lithium could be utilized in astronomical gamma-spectrometers, nuclear reaction spectrometers, and plasma physics X-ray instruments; Si-Ge10 and single crystal germanium doped with Hg, cadmium, copper and gallium are used in infrared detectors.
The infrared transmission range of Germanium is a staggering 2000-15000nm, spanning the Near-infrared to the Long Infrared Wavelength Range. And it can effectively resist corrosion from the atmosphere and the surrounding environments due to its high chemical durabilities, hence the Infrared windows made of germanium exhibit great reliability and performance in shielding the Infrared facilities from harsh environments and extreme conditions.
The high refractive index of Germanium makes it an attractive candidate for manufacturing infrared optical lenses. With the high refraction of the germanium lens, shorter focal lengths, and stronger converging power could be realized with fewer lens numbers, making the design more compact and simple. The large dispersion of Ge also makes a great material for infrared dispersive prisms.
Bismuth germanate is applied in scintillation radiation detectors. Glass made of germanium dioxide has a high refractive index and dispersion performance, and can be used for wide-angle cameras and microscope lenses; GeO2-TiO2-P2O5 type glass has outstanding infrared performance, and can be used to protect ultra-sensitive infrared detectors in space technology.
The Fabrication Of IR-grade Single Crystal Germanium
There are two methods to prepare germanium single crystals: one is the Czochralski method, and the other is the zone melting method:
① The procedure of the Czochralski Method is: a). Place a germanium ingot in a crucible to melt it; b) use a seed germanium crystal fixed on the rod, immerse it along the vertical direction in the molten germanium whose temperature is a little bit higher than the melting point, and pull it out from the melt at a certain speed, the molten germanium will freeze according to the crystallization direction of the seed crystal. Through controlling the pulling speed, the crucible, the rotation speed of the seed crystal speed, and other measures, as well as the techniques such as automatic adjustment of furnace temperature and single crystal diameter, n-type Ge with a resistivity of 0.003-40 ohm-cm and p-type Ge with resistivity of 0.002-40 ohm-cm, of a dislocation density of 500-3000 cm2 and a diameter of 20-300 mm can be obtained.
② The furnace used in the Zone Melting Method is a horizontal quartz tube heating furnace, which can produce germanium single crystals with uniform resistivity. The radial uniformity of resistivity is ±3%, the longitudinal uniformity is ±7%, the dislocation density It is 103 cm-2, and the single crystal section is 5-12 cm 2.
Germanium Windows:
Optical Windows are transparent optical components designed to protect the core of optics from detrimental damages from exterior environments such as dust, moisture, and contaminants, whilst transmitting light with minimum absorption, distortion, or deflection.
The application zone of optical windows stretches from general manufacturing, aviation, thermal imaging, plasma chambers, sensors, lasers, etc. Optical windows secure a stable optical interface and facilitate the accurate propagation of light.
Germanium Windows has a compelling infrared transmission of 2000-15000nm, whilst being entirely opaque to visible light, which is a nuisance to infrared devices. Furthermore, as mentioned above, Ge has a medium hardness value, therefore it could be fabricated and polished with a fat lot of difficulties, polishing it can be just as simple as polishing a glass material. And as Germanium features extreme chemical inertia, it can protect the IR facilities from the potential harm resulting from climates and harsh conditions. Germanium windows can also be coated with Diamond-Like-Carbon (DLC) coatings to form exceptionally tough optic fronts. However, Germanium has a large density, so caution should be taken where weight is an issue in the design, and since its refractive index is large, AR coatings are needed when Germanium is to be hired as an optical window.
It is also worth noting that germanium is toxic, so wear a pair of gloves while handling it, and wash your hands when you finish working.
Hangzhou Shalom EO is a professional supplier and manufacturer of Infrared optics, including Germanium Windows. We offer 23 modules in stock with a maximum diameter of 101.6mm, while other custom versions with a maximum diameter of up to 300mm could also be tailored upon your request. Coating options include diamond-like-carbon (DLC) coating, AR coatings (3-5 micro, 8-12 micro), and BBAR coatings ( 3-12 micro).
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